In order to add to the understanding of the neural processing of complex sounds, population studies of the responses of chinchilla auditory-nerve fibers to speech and speechlike sounds are proposed. Much information about the encoding of synthesized vowels with varying degrees of complexity and of selected consonants has recently become available. There is less information about the representation of other consonants, particularly those differing in voice-onset time. In one series of experiments, sounds in which acoustic correlates of the voicing feature are systematically varied will be studied. To aid in the interpretation of the data, the speech stimuli will be selected from those for which behavioral discrimination has previously been measured for the chinchilla. Comparison of neural and psychophysical responses to the same sounds and in the same species may provide insight into the kind of neural code that could transmit adequate spectral information to the central nervous system (CNS). This is so because the results of psychophysical tests define the limits of the chinchilla's ability to process human speech sounds and therefore provide an estimate of the amount of spectral information that should be observable in neural responses. In a second series of experiments, responses of auditory nerve fibers will be obtained in response to synthesized speechlike sounds in which important acoustic properties are parametrically varied. Previous studies of speech encoding in the auditory nerve have emphasized the contribution of nonlinear auditory processing to the observed response patterns. However, it has also been noted that responses to certain sounds or spectral regions are less affected by nonlinear mechanisms. A systematic exploration of the stimulus parameters that result in nonlinear transformations is lacking. Studies of auditory nerve fiber responses to harmonic complexes, in which the amplitudes of selected components can be independently varied, will be conducted. The results of these studies will provide additional specific information about the effects of nonlinear transformation on the extraction of these important spectral features.